Lanzmann decided to sell the letters because French inheritance law, which he called "scandalous," would have meant they would be bequeathed to de Beauvoir's family after his death.

However, he is entitled to "to pass them on in the hope that the purchaser can, if not publish them, then at least conserve them and make them available to historians and researchers."

"I never planned for these letters to come out or be published," said Lanzmann, noting that the "crazy" law "states that the contents of the letters did not belong to the person they were addressed to."

The intimate correspondence now occupies pride of place in the library of the distinguished US university in New Haven, Connecticut, along with de Beauvoir's manuscripts and personal archives, where they can be viewed only in the reading room.

Lanzmann, then 26, was secretary to Jean-Paul Sartre, 44-year-old de Beauvoir's most famous lover, when their seven-year affair first began in the 1950s.

Not as scandalous as it might sound: Sartre and de Beauvoir maintained an open relationship and both had multiple partners, although Lanzmann was the only one de Beauvoir would ever share a home with.

The letters, said to be filled with "mad passion," were mostly written while de Beauvoir was traveling in China, Russia, Japan and Cuba with Sartre, and Lanzmann calls them "an exceptional, passionate correspondence."

Agnes Poirer, author of "Left Bank" – a book about the French intellectual scene in the 1940s – says Lanzmann and de Beauvoir "had two little desks and they would work together in the mornings, then in the afternoons she would go and write with Sartre."

"After the age of 40, de Beauvoir thought she was not desirable any more, but she had a second youth with him."

Lanzmann, now 93, fell in love with de Beauvoir while he was editing "Les Temps Modernes," the journal she had founded along with Sartre in 1945. He remains the journal's editor-in-chief to this day.

His most acclaimed film is nine-and-a-half-hour-long documentary "Shoah," featuring interviews with victims and perpetrators of the Holocaust.